This invention relates to a mask frame for a cathode ray tube and particularly to a mask frame which can reduce the affect of electron beam paths by an external magnetic field, and allows distinct color reproduction.
Especially, the mask frame reduces change in the paths of the electron beams and cuts down the amount of change in the magnetic field of the mask frame corner parts which is caused by changes in the exterior magnetic field such as the earth's magnetic field.
The prior art color cathode ray tube allows color reproduction and makes it so electron beams shot from an electron gun land precisely on phosphor material of three colors which is spread on the inner surface of a panel.
Generally, a color cathode ray tube is comprised of a bulb having a panel on which a shadow mask is disposed which is a means for color reproduction, a funnel disposed as a tending means to construct raster and control the electron beam passing the interior of the CRT, and a neck in which an electron gun is disposed.
The shadow mask is adhered by welding etc., to the mask frame, which is supported on a skirt portion of the panel through a bimetal spring. An inner shield, which protects electron beams from the earth's magnetic field, is adhered to the rear of the mask frame.
FIG. 2 is a sectional view showing the panel 2 part of the color cathode ray tube having a mask frame 4 inside the skirt part of the panel supported by being suspended by a bimetal spring 6.
A shadow mask 8, having formed a great number of slits and has basically an identical curvature with the inside of said panel 2, is adhered to the front part of the mask frame 4.
The mask frame is produced in a rectangular shape of constant width and a flange part 12 to attach the mask frame 4 to an inner shield 12, and provided toward the back of the mask frame.
That is, FIG. 3 shows a mask frame 4 which is manufactured using an aluminum alloy metal or by a metal with invar qualities.
A mask assembly with this type of structure is able to diminish any influence the paths of the electron beams might receive, and is able to isolate the earth's magnetic field by the inner shield 10 when electron beams go toward the panel.
Especially, such as described in FIG. 3, an extreme change in the magnetic field results in the inner corner part of the mask frame because the structure of the corner part, realized through the meeting of a long side part 16 of the mask frame with a short side part 14 is made in a complete plate body, and there is an increase in the area that leads to a decrease in resistance.
But in a mask frame assembly structured as in the above, because the vertical elements of the earth's magnetic field operate from top to bottom in the northern hemisphere and the level elements of the earth's magnetic field operate from right to left, the magnetic field formed in a corner part of the mask frame is changed by these vertical and level elements of the earth's magnetic field.
Particularly, while this change is most extreme at the mask frame corner part, as a screen corner part's display characteristics are reduced in quality, a phenomenon arises wherein there is more of a decline in colorimetric purity of the corner part of the display than in the central part.
The reason for this extreme change of magnetic field in the inside corner part of the mask frame is because when the earth's magnetic field flows from top to bottom, the path is changed by a long side of the mask frame, and as it follows a short side of the mask frame, the vertical elements of the earth's magnetic field come to flow diagonally in the corner part of the mask frame.
This flow in a diagonal path is known to affect the path of electron beams passing this area.
The present invention has been made to solve the above problems.
It is an object of the present invention to provide a mask frame for a cathode ray tube that improves the colorimetric purity of a screen corner part and minimizes changes in a magnetic field of a mask frame corner part caused by changes in the earth's magnetic field.
To achieve the above objective, in a color cathode ray tube having a shadow mask to allot electron beams to each phosphor material inside the panel, the mask frame for a cathode ray tube is provided to distinguish the forming of resistance holes to the corner part joining a long side of the mask frame with a short side thereof.
Said resistance holes are provided for the CRT mask frame the same number at each side.
Also, a plurality of said resistance holes are provided at a flange corner part of the mask frame.